How Circadian Rhythms Give Vegetables A Healthy Boost

Researchers at Rice University conducted lab studies using light-dark cycles to try to coax more beneficial compounds out of fruits and vegetables.

Heather Rousseau
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Originally published on June 21, 2013 3:56 pm

Just as we have internal clocks that help regulate the systems in our bodies, fruit and vegetable plants have circadian rhythms, too.

And a new study published in Current Biology finds there may be a way to boost some of the beneficial compounds in plants by simulating the light-dark cycle after crops are harvested.

So, how does it work?

Well, take cruciferous vegetables such as cabbage, which contains cancer-fighting compounds called glucosinolates. Studies have shown that glucosinolates secrete enzymes that can remove carcinogens.

"The protective effect of these vegetables is that they rev up the capacity of cells to dispose of toxic compounds," says Paul Talalay, a pharmacologist at Johns Hopkins University.

In other words, they help our bodies get rid of harmful substances.

Now, as vegetables go from the field to the store to our plate, the levels of these compounds start to fizzle out.

If you listen to my story on All Things Considered, you'll hear how researcher Janet Braam of Rice University and her colleagues conducted lab studies to test the possibilities of coaxing more life — and more of these beneficial compounds — out of the fruits and vegetables we buy.

They put cabbages under light for 12 hours a day, followed by 12 hours of darkness, to try to re-create the light-dark cycle in the field.

Prior research has already shown that plants use circadian rhythms to help them judge when to turn on their chemical defenses. Some plants release these beneficial chemicals to fend off bugs in the field or cope with the stresses of heat or drought.

"And sure enough we found that when we put the plants under light-dark cycles, we found periods of accumulation of those [beneficial] chemicals," explains Braam.

The peak of the compounds came in the afternoon, in the hours before dusk.

They were "significantly higher in the day," Braam says — about twice as high. It was as if the plants were still alive, even though they're no longer attached to their roots or the earth. "This very much surprised us," Braam says.

Her team found similar responses with a range of crops including lettuce, spinach, sweet potatoes and blueberries.

Outside experts seem impressed by the study. "I'm excited to see this research going on," plant researcher Bhimu Patil told me by phone. And he explains there's a lot of interest in the industry in figuring out ways to retain the beneficial compounds in produce after harvest.

Braam is quick to point out that more research is needed to see whether the findings carry over into real-life situations, like our kitchens or in grocery stores. "It just opens the door to many possibilities," says Braam.

For instance, it might make sense for supermarkets — or consumers at home — to think abut storing our produce under light-dark cycles.

Or maybe it's time for a vegetable happy hour: eating our produce in the hours before dusk when some of the most beneficial compounds are at their peak.

Copyright 2013 NPR. To see more, visit http://www.npr.org/.

Transcript

ROBERT SIEGEL, HOST:

Now, a far less contentious story about the food that we grow. Scientists are trying to understand the secret life of vegetables, including cabbages, sweet potatoes and leafy greens. Understanding these secrets may help us get more nutritional bang for our buck.

NPR's Allison Aubrey has this report on some new research.

ALLISON AUBREY, BYLINE: It's not news that fresh produce contains lots of healthy compounds or that cruciferous vegetables - like cauliflower and cabbage - are nutritional superstars. But why are these plant chemicals so good for us? And why are they there in the first place? To make sense of this, I have dragged my colleague Dan Charles, who covers agriculture, along with me to the grocery store.

DAN CHARLES, BYLINE: Cabbage, cabbage, cabbage to our left.

AUBREY: All right.

Now, this is kind of hokey. But what I'm asking Dan to do is to pretend that he is a cabbage, to sort of channel his inner cabbage.

You ready, Dan?

CHARLES: I've got my cabbage.

AUBREY: All right, you wear it well.

Now, it turns out that all these good compounds are produced because plants have a pretty tough life out in the field. There are bugs to battle, severe heat and cold. And plants can't just get up and walk away from these challenges.

CHARLES: It's a problem because I'm a cabbage. I've got no hands to slap insects. I can't put up an umbrella to protect myself from the sun. I'm stuck. I'm stuck in the ground.

AUBREY: Yeah, I can see that. Your options are very limited here.

CHARLES: But what I can do is I'm a plant, what I do is I make chemicals. So what I and what my ancestors have done for millennia is to create chemical compounds inside myself to protect myself, to make myself unappetizing to insects, to detoxify the effects of the sun.

AUBREY: Now, it turns out that scientists have discovered that some of these same compounds that help plants defend themselves are good for us too.

PAUL TALALAY: We have studied at great length how these compounds protect human cells.

AUBREY: That's Paul Talalay of Johns Hopkins University. He says that compounds called glucosinolates found in cabbages and other cruciferous vegetables work like little detox agents in our cells.

TALALAY: What the protective effect of these vegetables is, is that they rev up the capacity of cells to dispose of damaging compounds.

AUBREY: In other words, they help our bodies get rid of toxins. Now, as vegetables go from the field to the store to our plates, the levels of these compounds start to fizzle out. So researcher Janet Braam of Rice University had a question: Was there a way for these plants to keep producing these beneficial chemicals after they'd been harvested?

JANET BRAAM: So we went to the grocery store and we just bought some cabbage.

AUBREY: And they tried to recreate some of the conditions in the field, in particular, the light-dark cycle.

BRAAM: So what we knew was that plants can use their circadian clock to turn on defenses at particular times of day.

AUBREY: So they put the cabbages under light for 12 hours a day, followed by 12 hours of darkness.

BRAAM: And sure enough, we found that when we put the plants under light-dark cycles so that they had good circadian rhythms, we found periodic accumulation of those chemicals.

AUBREY: Especially in the afternoon, in the hours before dusk.

BRAAM: Generally, they're significantly higher in the day than they are at night.

AUBREY: It was as if the plants were still alive even though they're no longer attached to their roots or the earth.

BRAAM: This very much surprised us.

(LAUGHTER)

AUBREY: Why?

BRAAM: We decided to do the experiment because we thought it was possible that it could work. And yet when it worked, I think we were all taken by surprise.

AUBREY: And they went on to find similar responses in more crops from lettuce and spinach to carrots and blueberries.

Now, Braam is quick to point out that these were just lab experiments. But if the findings carry over into real-life situations, they could have implications for how and when we harvest and store produce.

BRAAM: I think it just opens up the door to many possibilities.

AUBREY: Braam says it might make sense for grocery stores or for us at home to think about storing our produce under light-dark cycles. Or maybe it's time for a vegetable happy hour, eating our produce in the hours before dusk when some of the most beneficial compounds are at their peak.